1. Field of the Invention
The present invention relates to a charge controlling agent and a negatively chargeable toner containing the charge controlling agent used for an image-forming apparatus for developing an electrostatic latent image in the fields of electrophotography, electrostatic recording and the like.
2. Discussion of Background
In an image-forming process by electrophotographic system, an electrostatic latent image is formed on a photosensitive material comprising an inorganic or organic material, developed by a toner, transferred onto a paper, a plastic film or the like, and fixed thereon to obtain a visible image. The photosensitive material has negative chargeability or positive chargeability depending on its structure, and when an image part is left as a latent image by exposure to light, the latent image is developed by a reversely charged toner. On the other hand, when an image part is destaticized to carry out reverse development, the development is carried out with a charged toner the same charge.
A toner comprises a binder resin, a colorant and other additives. In order to impart a preferable chargeability (charging speed, charging level, charging stability or the like), a stability as a lapse of time, an environmental stability and the like, a charge controlling agent is generally added to the toner. Properties of the toner are greatly improved by adding the charge controlling agent.
Examples of a charge controlling agent having an effect of imparting a negative charge controlling property conventionally proposed and practiced, include a monoazo metal complex compound having chromium as a center metal, a monoazo metal complex compound having iron as a center metal, and a metal complex of an alkyl salicylic acid or aromatic carboxylic acid or their salt.
However, these charge controlling agents have such defects that a tribo-chargeability or an affinity to a binder resin of a toner is unsatisfactory, that a quick chargeability (charge up speed) is unsatisfactory, that an initial copy image is not clear, and that a quality of a copy image is easily variable during continuous copying operation. Also, a charge controlling agent comprising a metal complex of an alkyl salicylic acid or aromatic oxycarboxylic acid or its salt has such defects that a toner chargeability is largely varied depending on environmental conditions and that an image quality is largely varied depending on a season factor.
Among monoazo chromium complex compounds, there is a compound solving a part of these problems, but there is a possibility that a minor amount of noxious hexavalent chromium is formed at the time of burning and discarding and it is worried to provide a bad influence on environments or human bodies. Accordingly, it has been demanded to provide a charge controlling agent using a safer metal and having satisfactory properties.
JP-A-61-155464 discloses a toner using a charge controlling agent containing iron as a center metal. Although this is a negatively chargeable charge controlling agent containing no chromium as a center metal, this charge controlling agent provides a practical charged amount (at least −10 μc/g) but a quick chargeability is poor as compared with a chromium complex and there is a problem that a chargeability is lowered under a high humidity environment. Also, JP-A-8-500912 discloses some azo iron complexes, but all of azo complexes having satisfactory performances have a plurality of nitro groups and therefore there are always risks of combustion and explosion during synthesizing these compounds. Particularly when the center metal is iron, a risk of occurrence of combustion and explosion is very high and drying and pulverizing steps are very dangerous operations. Also, since a pulverized type toner is generally kneaded by an extruding kneader and is pulverized, there is a possibility of causing powder explosion at the time of preparing the toner. When using chromium at a center metal, a possibility of causing combustion and explosion is lowered as compared with iron, but an azo chromium complex having a nitro group obtained in this case corresponds to a self-reactive material (Class 5 dangerous material).